A universal serial bus interface (USB) which is applicable for an electronic device (such as, computer) is connected to a signal transmission interface of a peripheral apparatus (such as, flash disk, keyboard, mouse, speaker and so on) so as to provide convenient and rapid connection ways. With advances in technology, larger transmitting bandwidth of the USB has been developed. The largest bandwidth of standard USB 1.1 is 12 megabits per second (Mbps), and that of USB 2.0 is 480 Mbps. Currently, the largest bandwidth of USB 3.0 is increased from 480 Mbps to 5 gigabits per second (Gbps).
The advances in USB also improve power supply for the peripheral apparatus. Take USB 2.0 for example, USB 2.0 not only transmit data in both directions but also supply power with 5 volts and 500 milliamperes. Moreover, applying USB 3.0, information transmitting rate may be up to 5 Gbps and power is supplied with 5 volts and 900 milliamperes. When a speaker having a USB 3.0 interface is directly connected to a USB 3.0 transmission port of a computer, the output power of the speaker is increased and the sound effect (namely, quality) is also improved.
The development of USB is to provide better effects (namely quality), but actual requirement also needs to be considered. Updating of USB version needs to provide backward compatibility (compatible with former version of USB). However, for most users, when the USB version is improved, most users may not clearly distinguish the transmission port of the computer or the transmission port of the peripheral apparatus. For example, the version of the transmission port of the computer is USB 2.0 and the transmission port of the speaker is USB 3.0. Because the USB 3.0 version allows backward compatibility, the USB 3.0 interface transmission port of the speaker may be inserted into the USB 2.0 interface transmission port of the computer. However, electrical circuits in the speaker are designed to match with USB 3.0 circuits with larger current inputting, such that current provided by USB 2.0 interface transmission port of the computer may not match with the USB 3.0 electrical circuits in the speaker.
It can be known from a relation of power (P), current (I) and voltage (V): P=IV, The optimized output power of the USB 3.0 speaker is 4 watts, when the USB 3.0 interface transmission port is inserted into the computer, the speaker may receive 4.5 Watts of input power (5 volts times 0.9 amperes), the optimized effect is achieved. When the USB 2.0 interface transmission port is inserted, the speaker may only receive 2.5 watts of input power (5 volts times 0.5 amperes). Therefore, voice cracks may occur and the volume of the sound generated by the speaker may be suddenly increased and decreased.